1. Field of the Invention
The present invention relates to an antiskid control apparatus and more particularly to an antiskid control apparatus to be applied to a part-time four wheels drive (4WD) in which a two wheels drive (2WD) can be switched to a four wheels drive (4WD) and vice versa.
2. Description of the Related Art
In applying the antiskid control apparatus to a vehicle adopting the 4WD, the wheel speeds of the vehicle are synchronized with each other especially on a road having a low friction coefficient because the axle of the front wheels and that of the rear wheels are connected with each other. Thus, in the general control of a 2WD vehicle, for example, in controlling the front wheels independently and controlling the rear wheels by select low control, each wheel slips greatly, thus lowering the stability of the vehicle and generating uncomfortable vibrations on the vehicle.
various antiskid control apparatuses for use in the 4WD vehicle have been proposed.
According to an antiskid control apparatus disclosed in Japanese Laid-Open Patent Publication No. 1-311939, when the difference between the average wheel speed of front wheels and the average wheel speed of rear wheels is large, the general control of the 2WD vehicle is performed, whereas when the difference between the average wheel speed of the front wheels and the average wheel speed of the rear wheels is small, i.e., when the wheel speeds of the vehicle are synchronized with each other, wheels belonging to the same hydraulic system are controlled by selecting the low control. In this manner, the wheels are prevented from interfering with each other.
As a method for driving a vehicle, part-time 4WD is known. In the part-time 4WD, the 2WD State in which the axle of the front wheels or that of the rear wheels is driven can be switched to the 4WD state in which the both axles are connected with each other and vice Versa. A disadvantage occurs when the conventional antiskid control apparatus for the 4WD is applied to the part-time 4WD.
That is, even when the 2WD state is selected in the part-time 4WD, there is a case in which the difference between the average wheel speed of the front wheels and the average wheel speed of the rear wheels becomes temporarily small. In this case, if wheels belonging to the same hydraulic system are controlled by select low control as in the case of the conventional antiskid control apparatus for use in the 4WD, the braking force is lowered and thus the stopping distance becomes long.
In the part-time 4WD, although the speed of the front wheel and that of the rear wheel are synchronized with each other in the 4WD state, phase difference occurs due to the torsional rigidity of the driving shaft (propeller shaft). As a result, the difference between the average wheel speed of the front wheels and the average wheel speed of the rear wheels fluctuates and may become temporarily large. In this case, in the conventional antiskid control apparatus for use in the 4WD, there is a case in which the control of the 2WD is performed although the vehicle is in the 4WD state. As a result, the wheels cannot be prevented from interfering with each other.
It is conceivable to provide a sensor for detecting the switching between the 2WD state and the 4WD state to change over the control of the braking system by inputting a signal outputted from the sensor to the antiskid control apparatus. The provision of the change-over sensor causes the manufacturing cost to be high and the antiskid control apparatus to be unreliable.